Question 1

What is the Meaning of Free Triiodothyronine (ft3)?

Accepted Answer

Free Triiodothyronine, abbreviated as fT3, represents the fraction of the biologically active thyroid hormone triiodothyronine (T3) that is unbound to serum transport proteins in the bloodstream. T3 is the most potent thyroid hormone, responsible for regulating metabolic rate, heart function, digestion, and muscle control across all body tissues. Because only the free fraction can enter cells and exert its effects, the fT3 level is considered the most accurate indicator of true thyroid hormone activity at the tissue level. Measuring fT3 provides critical diagnostic information about a patient's thyroid status, independent of fluctuations in binding protein concentrations.

Question 2

What is the Origin of Free Triiodothyronine (ft3)?

Accepted Answer

The term is derived from the chemical structure of the hormone, triiodothyronine, which contains three iodine atoms, and the physiological state of the molecule in the blood. The prefix "Free" (f) distinguishes this metabolically available fraction from the total T3, which includes the much larger protein-bound reservoir. This distinction became necessary in clinical endocrinology to accurately assess thyroid function in the presence of altered thyroid-binding globulin levels, such as during pregnancy or estrogen therapy.

Question 3

What is the Mechanism of Free Triiodothyronine (ft3)?

Accepted Answer

Triiodothyronine, whether secreted directly by the thyroid or converted from T4 by deiodinase enzymes, circulates primarily bound to proteins like thyroxine-binding globulin (TBG). This binding acts as a reservoir and prevents rapid metabolism. The small, unbound fT3 fraction is the only form capable of diffusing across the cell membrane to bind to the nuclear thyroid hormone receptor (TR). Upon binding, the T3-TR complex modulates gene transcription, thereby initiating the diverse metabolic and developmental actions of thyroid hormone throughout the body.

Free Triiodothyronine (ft3)

Meaning

Origin

Mechanism

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